This invention relates to the measurements of surface properties in semiconductors and, more particularly, to a method to determine the electron affinity and to simultaneously locate the position of the conduction-band edge with respect to the Fermi level at the surface of single crystal-semiconductors.
In determining semiconductor surface properties, one of the most surface-sensitive techniques is the measurement of the surface work function .phi.. However, this quantity is composed of two components; (a) the position of the conduction-band edge E.sub.c with respect to the Fermi level and (b) the electron affinity .chi.. The former is governed by such band structure properties of the solid as the donor position, density, and ionization probability, while the latter is strongly dependent on characteristics such as surface dipole layers. Because of these differences, it would be desirable to separate the two components in the measurement of work function. This will not only determine the various properties simultaneously for a particular set of surface conditions, but will also quantitatively identify changes in work function as being caused by specific contributions from either or both of its components.
In the past, there has been no singularly direct and reliable method for experimentally determining these two components of the work function despite a great deal of interest in the area. For example, alkaline earth semiconductor compounds (such as barium oxide) have been widely used to provide low-work-function surfaces for electron emission applications and have been extensively studied for many years. However, there are still many basic but yet unanswered questions regarding which factors are actually responsible for the activity of these alkaline earth oxide emitters. Investigation of many of these questions would be aided if the work function could be separated into its components, which as previously noted are strongly influenced by specific surface properties.
Experimentally, electron affinity has been inferred from combining the results of separate procedures. For example, the electron affinity .chi. has been estimated from the temperature at which pore conductivity and crystal conductivity are roughly equal in oxide cathodes. It has also been derived by combining optical adsorption and photoconductivity measurements. These techniques, in addition to requiring separate experiments (using different apparatus so that the experiments cannot be accomplished simultaneously without great difficulty), also involve assumptions that give uncertainty to the estimated values, and are not generally applicable to all semiconductors. Conductivity, in general, may be measured by mechanical probes, but this leads to the problems which are normally associated with probe measurements. These are especially acute when it is desired to measure the conductivity for small surface areas in the region near the semiconductor surface.